The present invention relates to an oxidation catalyst which is resistant to sulfur oxides. In particular, the present invention is directed to an oxidation catalyst useful in purifying exhaust and waste gases and, more specifically, to an oxidation catalyst capable of converting carbon monoxide and hydrocarbons to carbon dioxide and water with high efficiency even in the presence of sulfur oxides.
The present invention also relates to a process for converting carbon monoxide and hydrocarbons such as contained in exhaust or waste gas streams to carbon dioxide and water even when the exhaust or waste gas streams contain sulfur oxide components. The invention is also directed to a process for converting sulfur dioxide to sulfur trioxide using the novel sulfur-resistant oxidation catalyst of the present invention.
Hydrocarbon and CO abatement in various waste and exhaust gas streams may be accomplished by reacting the waste or exhaust gas with air over a platinum-containing catalyst. Typically used at present is a catalyst similar to the auto-exhaust catalyst which is comprised of platinum supported on alumina supports or alumina-containing supports which are often deposited over a ceramic honeycomb. Such catalysts are effective oxidation catalysts and deactivation of such catalysts are minimal when the waste or exhaust gas is devoid of sulfur oxides, to be referred to as SO.sub.x, including SO.sub.2. However, the exhaust and waste gases of many industrial operating systems including those cogenerating electricity and steam which are powered with diesel fuel or refining gas often contain up to a few hundred parts per million of SO.sub.2. Under reaction conditions, SO.sub.2 oxidizes to SO.sub.3 which in turn reacts with alumina to form aluminum sulfate and, thus, renders the alumina-containing oxidation catalysts inactive.
It is known to use oxidation catalysts which contain catalytic metals on refractory oxide supports other than alumina. Thus, supports comprising silica, titania, zirconia and mixtures of these oxides are known and such supports including the binary and ternary mixtures of the above oxides are known to be resistant to SO.sub.x. Unfortunately, it is also known that it is difficult to anchor a precious metal such as platinum on a silica support. The weak interaction between silica and precious metals results in severe precious metal sintering at very moderate conditions drastically reducing the surface area and, thus, activity of the precious metal catalyst. Moreover, although titania and zirconia interact with precious metals to significantly reduce sintering of the precious metal, the high initial surface area of both zirconia and titania (anatase) is drastically reduced after calcination at 500.degree. C. The loss in surface area of zirconia and titania at typical reactive oxidation conditions is unfavorable inasmuch as the loss in surface area results again in the sintering of the precious metal and deactivation of the catalyst.
Accordingly, it is an object of the present invention to provide an effective carbon monoxide oxidation catalyst.
It is another object of the present invention to provide an effective carbon monoxide oxidation catalyst which is stabilized against deactivation at the elevated temperatures of reaction.
It is another object of the present invention to provide an effective carbon monoxide oxidation catalyst which is stabilized against deactivation by the presence of SO.sub.x.
It is a further object of the present invention to provide an effective oxidation catalyst which can effectively oxidize gaseous sulfur-containing compounds to SO.sub.2, SO.sub.3 or mixtures thereof.
Yet another object of the present invention is to utilize a sulfur-resistant carbon monoxide oxidation catalyst in a process for purifying waste and exhaust gas streams of hydrocarbons and carbon monoxide.
Yet still another object of the present invention is to provide a process for effectively oxidizing SO.sub.2 to SO.sub.3 in the presence of a stabilized oxidation catalyst.